1. Field of the Invention
The present invention relates to a photocathode that is responsive to wavelengths within the range of 0.9 .mu.m to at least 10 .mu.m. More specifically, it relates to the combination of an infrared (IR) absorbing semiconductor (GaSb, InAs, or a superlattice of Ga.sub.w In.sub.y Al.sub.1-y-w Sb and InAs) with an electron emitter made of Ga.sub.x Al.sub.1-x Sb.
2. Description of Related Art
The general concept of a semiconductor-based, infrared-sensitive photocathode was published in the 1970's. Prior to this, the "standard" S-1 metallic photocathode was usable for light with wavelengths out to about 1.3 .mu.m, but the S-1 photocathode is plagued with high noise levels and relatively low quantum yield. With the invention of the p-type GaAs photocathode [J. J. Scheer and J. Van Laar, Solid State Comm., 3, 189 (1965)], excellent performance was demonstrated for light with wavelengths out to about 0.9 .mu.m. In general, semiconductor photocathodes are p-doped. This causes downward band bending at the cesiated surface and causes ballistic ejection of the conduction-band electrons through the Cs/CsO into the vacuum. The p-doped GaAs photocathode has the highest quantum efficiency of any photocathode for visible and nearest-IR light. For light between 0.9 .mu.m and 1.3 .mu.m, the S-1 photocathode still had to be used.
Two general approaches which use semiconductors to improve their quantum efficiency and IR response compared to the S-1 photocathode have been proposed. In the first approach, a single material both absorbs the light and emits the photogenerated electrons into the vacuum, but the bandgap of the semiconductor was simply lowered by epitaxially growing In.sub.x Ga.sub.1-x As or GaAs.sub.1-x Sb.sub.x, with small x, on GaAs, for example, as demonstrated in U.S. Pat. No. 3,814,996 (1974) by Enstrom and Fisher. Unfortunately, lowering the bandgap in this way below about 1.4 eV quickly kills the efficiency of the electron emitter, so this simple approach did not extend the sensitivity to light with wavelengths longer than 1.3 .mu.m.
The second approach was to use externally applied electric fields on the semiconductors in the photocathode to force the electrons to move from one physical region to another and to transfer them from the lowest conduction band, .GAMMA. (gamma), to the next higher band, L or X, where the electrons would have enough energy to escape into the vacuum. The earliest and simplest example of this was in germanium, which showed 10.sup.-6 electrons/photon with a bias of 6 volts applied [R. E. Simon and W. E. Spicer, J. Appl. Phys., 31, 1505 (1960)]. This configuration was improved by fabricating the photocathode with at least two semiconductor components comprised of an IR absorber such as InGaAsP, and an InP electron emitter. By applying an electric field, the electrons were transported from the IR absorber and promoted from the .GAMMA. to the L conduction band [R. L. Bell, et al, Appl. Phys. Lett., 25, 645 (1974); J. S. Escher, et al, CRC Critical Reviews in Solid State and Materials Science, 5, 577.(1975).]. With this approach, photocathodes with quantum yields of greater than 0.1% to wavelengths as long as 1.4 .mu.m have been demonstrated [See the review article by W. Spicer, Applied Physics, 12, 115 (1977)].
Solid-state photodetectors which used electron transport across compositionally different materials have been fabricated at Rockwell Science Center [R. Sahai, et al, CRC Critical Reviews in Solid State and Materials Science, 5, 565.(1975)]. In previous work, photocathode devices were cooled to as low as -100.degree. C. in order to reduce the dark current. For devices which operate in the IR region, the need for some cooling can be expected. In an ideal device, the thermal generation of electron-hole pairs within the IR absorber would be the biggest source of dark current. The positively biased contact on the emitter, used to form the internal electric field, also can inject holes into the photocathode. This has several detrimental effects including non-uniform electric fields across the surface of the photocathode, impact generation of electron-hole pairs, and loss of signal electrons as they are transported from the IR absorber to the surface. Hole injection can be reduced by making a Schottky surface contact. Hole blocking layers composed of materials with a valence band offset with respect to the emitter could also be inserted between the electron emitter and the IR absorber.
From the prior art, it follows that in general, a single-crystal semiconductor device needs at least the following two components to work as an infrared-sensitive photocathode (IR-photocathode):
1. An IR absorber, i.e. a semiconductor which absorbs the IR light and promotes an electron into its conduction band in the process of IR absorption and; PA1 2. An electron emitter, i.e. a material which receives the electron from the conduction band of the IR absorber into its own conduction band (using an applied voltage bias) and then ejects the electron into vacuum.
The device may also need a graded (i.e. linearly) region between the IR absorber and the electron emitter to facilitate the electron transport to the surface.
When an absorber is used which has a smaller bandgap than the substrate, the photocathode may be used in a transmission mode where the photons to be detected pass through the substrate to the absorbing layer. The photogenerated electrons move toward the emitter and are ejected into a vacuum. The substrate acts as an optical filter by absorbing photons with energy greater than the bandgap of the substrate. In the case of GaAs or GaSb substrates, none of the visible wavelengths is transmitted to the absorbing layer, yielding a "solar-blind" photocathode. The other common mode of operation is where the photons to be detected pass through the emitter surface and into the absorber. This is called the reflection mode of operation.
Using a gallium antimonide (GaSb) or gallium arsenide (GaAs) substrate, one can grow InAs/Ga.sub.w In.sub.y Al.sub.1-y-w Sb superlattices (InAs stands for indium arsenide) which absorb IR light (wavelengths from 1.7 .mu.m to 10 .mu.m and longer have been reported). It is also known that GaSb, which absorbs IR wavelengths out to 1.7 .mu.m, or InAs, which absorbs IR wavelengths out to 4 .mu.m, can also be used as the IR absorbing component. It is desirable to combine these IR absorbing components with an electron emitter made of Ga.sub.x Al.sub.1-x Sb to form a photocathode sensitive to wavelengths within the range of 0.9 .mu.m to at least 10 .mu.m. The present invention provides such an article.